1. Field of the Invention
The present invention relates to a control system for an internal combustion engine and, particularly, to a control system which estimates a property of a fuel in use.
2. Description of the Related Art
Japanese Patent Laid-open No. 2005-171818 (JP '818) discloses a control system for a compression ignition internal combustion engine in which a premix combustion is performed. According to the disclosed control system, an actual ignition timing of fuel is detected when the premix combustion is performed, and a property of the fuel in use is determined according to an ignition timing error and differences in the ignition timing error. The ignition timing error is a difference between the detected actual ignition timing and a previously set reference fuel ignition timing.
When determining a fuel property based on a combustion state of the injected fuel, it is preferable to maintain the fuel injection amount at a constant value in order to avoid degrading the accuracy of the determination. However, there is a possibility that the combustion state may change due to a load applied to the engine from an air conditioner or auxiliaries influencing the combustion state. Such a situation would likely reduce the accuracy of the determination since the engine operating region where the premix combustion is performed is a low-load operating region where a demand torque TRQ of the engine is comparatively low. An example is shown in FIG. 7.
Further, the operating region shown in FIG. 7 is comparatively narrow compared with the entire engine operating region. Therefore, the inventor of the invention disclosed herein studied the issue of determining fuel property during an idling state of the engine. A result of the study is that there is a problem described below. That is, it is necessary in the idling state of the engine to control the fuel injection amount so that an engine rotational speed is substantially kept constant. With respect to the cylinder in which the estimation of fuel property is performed, estimation accuracy of the fuel property will decrease if the fuel injection amount is not maintained at a constant level. In this case, when using one cylinder for estimating the fuel property, for example, in a four-cylinder engine, it is necessary to keep the engine rotational speed substantially constant by controlling the fuel injection amount in the remaining three cylinders. If an air conditioner and/or the like driven by the engine are/is operating, the difference between a torque generated by the cylinder which is subjected to the fuel property estimation and the torque of the other cylinders becomes great in order to keep the engine rotational speed substantially constant, which also causes unpleasant vibrations.